In the fin tube type heat exchangers such as air conditioners and water heaters, heat exchange is performed by inserting the heat transfer tube, which is for passing refrigerant in the aluminum fin material, into the fin tube type heat exchanger. Conventionally, copper tubes have been used as the heat transfer tube. However, due to the demands for weight saving, cost reduction, and improved recyclability, there is a need for substituting the copper tubes with aluminum alloy tubes.
Recently, improvement of the heat-transfer property is attempted in air conditioners for energy saving; and re-examining the kind of the refrigerant and improvement of the structural design of the heat exchanger are made. Under the circumstance, there is a demand for a heat transfer tube, which is one of components of the heat exchanger, having an even higher performance. Currently, inner grooved tubes, on the inner surface of which continuous spiral grooves are provided, are mainly used to improve the heat-transfer property.
As a method of producing an inner spiral grooved tube, a groove rolling method, in which a tube is drawn while spiral grooves are rolled in the inner surface of the tube, is known (Patent Literature 1 (PTL 1)). In the groove rolling method for the conventional copper tubes, spiral grooves are rolled on the inner surface of the tube by pressing the tube on a grooved plug, which is provided in the inner circumference of the tube, from the outer circumference of the tube by using a ball bearing rotating at a high speed. Similar groove rolling methods are planned to be used for producing an inner spiral grooved tube made of aluminum or aluminum alloy.
As an alternative method of producing an inner spiral grooved tube, a method, in which an inner spiral grooved tube with an twist angle is produced by: using a raw tube having straight grooves on the inner surface; and drawing the tube with a drawing die while the diameter of the tube is reduced with the drawing die concurrently with twisting the raw tube in front of the drawing die to form plastic flow in the reduced diameter portion of the raw tube, is known (Patent Literature 2 (PTL 2)).
As another alternative method of producing an inner spiral grooved tube, a method, in which an inner spiral grooved tube is produced by: winding a raw tube, on the inner surface of which straight grooves along a longitudinal direction of the raw tube are formed with an interval in the circumferential direction, in a coiled shape; and stretching the coiled shape raw tube in a straight tubeshape by applying a constant tension along the axis of the coil to introduce twist in the raw tube, is known (Patent Literature 3 (PTL 3)).